This invention relates to a vinyl chloride free resinous composition useful in a hot melt process for gasketing closures, and to closure sealing gaskets made from such compositions by a hot melt process.
In the bottling of carbonated beverages and the like, an air tight closure is applied over the opening of the container, for instance, the mouth of a bottle, to retain the carbonation of the contents and to protect the contents from contamination. Crowns for such bottles are made of metal having uniform ductility and even temper and are crimped in place. Recently, a second type of bottle closure has been developed known as the roll-on closure, which comprises a soft aluminum, generally closed end cylindrical cap which is rolled onto the bottle neck, in the process being deformed to define threads. To provide an air-tight seal, these types of closures are lined with a sealing gasket consisting of cork, polyethylene, polyvinyl chloride (PVC), or other plastic material.
Another type of closure gasket is formed on the underside of caps designed for use on wide-mouth bottles used for packaging spreads, fruits and vegetables, mayonnaise, condiments and the like. These gaskets typically take the form of an annular ring positioned about the circumference of the interior of the cap which seals against the lip of the jar when the cap is in place. Such annular gaskets may be used to seal crowns, roll-on caps, CT (screw) caps, lug caps, and other caps.
To be effective, such gaskets should, preferably, be adhered to the interior of the closure and must be capable of uniformly contacting the mouth of the bottle. In this regard, it is imperative that the closure be elastomeric and soft enough so that is can conform to the shape of the mouth of the jar or bottle and so that no passage remains through which liquid or gas can pass. Particularly in the case of roll-on closures and wide-mouth bottle caps, since they are designed for reuse, it is necessary that the gasket or liner have some elasticity and some permanent set so that the seal can be reformed after the closure has been initially removed. In addition to these requirements, it is important that the liner be made of a material which will not impart any undesirable odor or taste and not contaminate the contents of the bottle. The ideal liner should also be stable when subjected to water, weak acids, alcohol, and temperatures to which it may be subjected during use, e.g., pasteurization temperature.
To be commercially successful, liners of the type described must not only have the above-recited properties, but also must be capable of being manufactured in a simple, inexpensive, and automated fashion. The most successful liner material in general use today has for its main ingredient a fluxed plastisol which basically comprises a dispersion of colloidal sized thermoplastic resin particles, usually PVC, in a liquid plasticizer in which the resin is insoluble or only slightly soluble at room temperature. In addition to the basic component, to accomplish conventional purposes, other ingredients such as extenders, fillers, pigments, stabilizers, wetting agents, and thickeners are included in the composition. Also, when a fluxed cellular liner is desired, gas may be dispersed in the plastisol or a blowing agent may be incorporated which will decompose at the fluxing temperature of the composition.
In use, the liner forming material is heated to about 46.degree. C, and a metered amount is forced through a nozzle of predetermined size into a closure shell which, typically, has a vinyl based epoxy or phenolic coating on its interior. The liner is then shaped by a molding punch heated to 175.degree. to 205.degree. C and transferred to an oven. Exposure to the oven temperature for between 7 to 60 seconds (depending on the type of oven) causes the resin to become substantially completely solvated by the plasticizer and yields a homogeneous solid solution which transforms itself to a rubbery mass on cooling. In an alternative procedure, the heated punch is eliminated and the liner is shaped by rotating the closure shell at high speeds. In the case of wide-mouth bottle caps, the gasket is shaped by rotating the closure shell while a ring of compound is laid down in the sealing area of the closure.
In a typical system of this type, operating to produce about 500 liners per minute, 100,000 BTUs per hour per line, excluding the fusing oven, are used, and roughly 250,000 BTUs per hour are consumed in each fusing oven alone.
One factor in the successful operation of a high speed liner system is the liner forming composition's viscosity. It is essential that a uniformly sized drop of the composition be deposited in each closure shell or a uniform bead of material be deposited in the sealing area of the wide-mouth bottle cap or crown. If the viscosity of the composition fluctuates because of variations in the temperature or other reasons, the size of the drop or thickness of the bead deposited will vary and a high frequency of rejects will result.
One drawback of the liner forming method described above is the relatively large amount of energy consumed in the lining process. Another drawback, the import of which cannot be ignored, is that vinyl chloride is a suspected carcinogen. The polyvinyl chloride dispersed in the plastisol is known to contain some unpolymerized vinyl chloride, and if trace amounts of the monomer are allowed to mix with substances intended for human comsumption, a potentially very serious health hazard results. It has recently been reported that the Food and Drug Administration has banned the use of PVC in rigid and semi-rigid food packaging items. The use of PVC in contact with any consumable substance may be regulated in the near future.
Accordingly, it is a principle object of the present invention to provide a hot melt composition for producing closure liners which contain no polyvinyl chloride and to thus eliminate the possibility that vinyl chloride monomer may contaminate the contents of the bottled substance.
Another object of the invention is to provide a process which is substantially more energy efficient than the plastisol process yet can be adapted for use in high speed liner forming systems.
Still a further object of the invention is to eliminate most of the air-polluting effluents normally associated with the fusing of plastisol closure liners or gaskets.
Still another object of the invention is to provide a method of manufacturing closure liners which can utilize much of the equipment presently used in the plastisol process.
Another object of the invention is to provide a gasket of the class described having all the desired characteristics of the prior art closure liners or gaskets which can be manufactured relatively inexpensively and at high speed.
Yet another object of the invention is to provide a liner which may be adhered to the interior of the closure shell by an inexpensive resinous coating instead of the more expensive vinyl based epoxy or phenolic resins used with plastisol liners.